1. Field of the Invention
This invention relates to optical instrumentation, and more particularly to a method and apparatus for depth profile analysis of materials by laser-induced plasma spectroscopy (LIPS).
2. Brief Description of the Prior Art
Coatings and surface modification by the diffusion of elements into materials are widely used in industry to give enhanced properties to the materials. A knowledge of the compositional variation in surfaces and interfaces is of primary interest since interfacial composition plays a key role in the functional behavior of the material. The quality of these layers can be investigated by a number of techniques depending on the information required. Classical analytical chemistry has focused on techniques and methods giving information on bulk composition and few are devoted to depth profiling. Techniques such as Auger and X-ray photoelectron spectroscopy have been used to study surface chemistry on the atomic scale, and can be used to probe into the coating by removing material through ion bombardment to yield depth profile data. Thicker coatings can be analyzed with the use of X-ray spectrometry and Rutherford backscattering techniques. However, such techniques require working in ultra-high vacuum conditions to avoid scattering by molecules in the gas phase, a circumstance that imposes severe restrictions on the practical use of these approaches. Glow discharge optical emission spectrometry (GD-OES) and glow discharge mass spectrometry (GD-MS) have been used to measure coatings over a thickness range 0.01 xcexcm to over 50 xcexcm. Measurement times are about 15 minutes and depth resolution is typically around 100 nm. These techniques suffer from poor lateral resolution. Furthermore the specimen shape and thickness is limited to the sample chamber configuration.
These and other conventional techniques used in industry for depth profile analysis require preparation of the sample, are time consuming, and involve high cost instrumentation (e.g. Auger, GD-MS). Furthermore, some techniques based on X-ray fluorescence are also limited in sensitivity.
An emerging method, laser-induced plasma spectroscopy (LIPS), promises to provide rapid, in-situ compositional analysis of a variety of materials in hostile environments and at a distance. Basically, this method includes focusing a high power pulsed laser beam on the material, thus vaporizing and ionizing a small volume of the material to produce a plasma having an elemental composition which is representative of the material composition. The optical emission of the plasma is analyzed with an optical spectrometer to obtain its atomic composition.
The great need in industry for fast techniques with on-site capabilities makes LIPS a promising technique for in depth profile analysis of layered materials. However, the energy distribution within the laser beam (typically a near Gaussian mode in many laser systems) has limited the depth resolution achievable with this technique as it produces cone-shaped craters with non-negligible edge contribution to the ablated mass. Several solutions have been proposed to remedy this problem. Vadillo and Laserna (J. Anal. At. Spectrometry, vol. 12, 1997, p. 859) improved the depth resolution of LIPS measurements by using a simple two-lens telescope combined with a pinhole mask to generate a collimated output of a XeCl excimer laser, resulting in a flat energy profile. Beam masking has also been employed to attenuate the shot energy and to eliminate the peripheral irregularity of the beam profile (by Kanicky et al., Fresenius J. Anal. Chem., vol. 336, 2000, p. 228). These approaches have solved, to some extent, the problem of irregular energy distribution over the beam cross section but have failed to eliminate the interaction between the laser and the wall of the crater. In fact, the plasma produced by the laser also interacts with the wall of the crater and induces some mixing of material, which complicates the analysis by LIPS, in particular in the region close to an interface.
An object of this invention is to provide a tool to overcome this problem and make it possible to realize a measurement without being affected by the edge of the crater.
It is also an object of this invention to enhance the resolution of depth profiling by LIPS. The basics of this technique are known in the art for analysis of elements present in a sample and is described, for example, in U.S. Pat. No. 5,751,416, the contents of which are incorporated herein by reference.
An object of our invention is to provide a reliable depth profile analysis of solid material. Accordingly, this invention consists in a new method and apparatus for measuring the evolution of concentration as a function of depth and can achieve a more accurate measurement than classical instrumentation, without sample preparation.
The present invention features two different probes. The same laser generates the two probes. The first probe produces a reproducible and controlled ablation that produces a first large crater and the second probe, collinear with the first, has a smaller beam size and allows generating the analytical plasma inside the crater. The emission of the plasma is collected and separated in an optical spectrometer.
Accordingly in a first aspect the present invention provides a method of spectrochemical depth-profile analysis of heterogeneous materials, comprising directing a first burst of ablation laser pulses in a first beam at a sample to form an ablation crater with a bottom and wall; directing a second single pulse or burst of laser pulses in a second beam having a smaller width than said first beam at the bottom of said crater so as to create a plasma that emits radiation representative of a component in the sample without significant contribution from the wall of the ablation crater, measuring the intensity of radiation from said plasma; determining the concentration of said selected component in said material from the intensity of said radiation; and evaluating the depth at which said plasma is created. The above steps are preferably repeated in order to determine the evolution of concentration of the selected component as a function of depth.
Many laser systems produce a near-Gaussian energy distribution within the laser beam, which limits the depth resolution achievable with the LIPS technique as it produces cone-shaped craters with a non-negligible peripheral contribution to the ablated mass. The first part of this invention allows obtaining a more homogenous ablation by using only the center of the laser beam. The laser shot number controls the ablation depth. The second part of this invention allows performing an analysis of the surface at the bottom of the crater, without any contribution from the crater wall.
In one aspect of this invention, there is provided an apparatus for depth spectroscopic analysis of heterogeneous materials, comprising an energy source for generating pulses of energy in the form of a first beam of predetermined width incident on a sample to cause ablation thereof and thereby form a crater with a bottom and a wall; an energy source for generating a single pulse or burst of pulses in a second beam of laser light, said second beam having a width less than said first beam and being directed at the bottom of said crater so as to form a plasma emitting radiation representative of a selected component present in said material without significant contribution from the wall of the crater; a detector for measuring the intensity of radiation of said selected component at different depths of crater; and a depth profile evaluator for determining the depth of the crater for each radiation intensity measurement.
The energy sources can be one or two lasers disposed such that their optical paths are substantially collinear. A small deviation from colinearity is acceptable.
The measuring device, e.g. a spectrometer, is preferably disposed substantially colinearly with the optical path of the laser beams.
The dimensions of the laser beam at the focal point is not a significant factor. The beam used for ablation must simply be larger than that used to carry out the measurement. Typically, a diameter ratio of 1/3 could be used.